A general method of developing catalytic DNA biosensors for on-site, real-time, and cost-effective detection and quantification of multiple heavy metal ions, such as mercury, cadmium and lead, simultaneously will be explored. Exposure to these metal ions can bring severe health effects to human beings, such as damages to brain and kidneys, and cancers. They can also cause adverse effects on neural and brain development in children and the effects in children are often irreversible. Therefore there is an urgent need of portable sensors for these metal ions for household uses, for clinical applications in rural or remote areas, and for first-responders in the war against terrorism, where on-site, real-time, and cost-effective detection and quantification are critical. This Phase I application addresses critical issues of how to apply a successful method in designing sensors for one metal ion to those of other metal ions, and how to enhance selectivity of those sensors. It is built upon a recent success in the PI's lab where highly sensitive and selective fluorescent and colorimetric sensors for lead have been demonstrated based on catalytic DMA. In phase I, the feasibility of applying this method as the general protocol to the two other heavy metals, namely mercury and cadmium, will be investigated. Specifically, a combinatorial biology technique called in vitro selection will be carried out to select Hg(ll)- and Cd(ll)-specific catalytic DMA. After negative selection strategies are used to improve the selectivity of the selected DNA molecules, fluorescent and colorimetric sensors for mercury and cadmium will be constructed and tested by attaching fluorophore/quencher and gold nanoparticles, respectively, to the catalytic DNA molecules. After achieving these goals in Phase I, catalytic DNA specific for arsenic and chromium will be selected in Phase II and the prototype sensor kits for these heavy metal ions will be developed for testing and marketing the sensors in environmental monitoring and medical diagnostic applications.